Operation of pulp digesters



Feb. 26, 1929. 1,703,745

a. MORTERUD I OPERATION OF PULP DIGBS'I'ERS Filed Feb. 25. 1927 2 Sheets-Sheet J.

Patented Feb. 26, 1929.

UNITED STATES EINAR MORTEBUD, OF TORDEROD, NEAR MOSS, NORWAY.

OPERATION OF PULP DIGESTEBS.

Application filed February 25, 1927, Serial .110. 170,941, and in Norway March 2, 1926.

The present invention relates to the digestion of wood pulp in closed digesters 'under pressure and has for its object a process by means of which it is made possible to utilize economically the hot gas and/or steam which has to be discharged from the upper part of the digester.

This dischargeof gas or steam takes place during the first part of the digesting process, for instance as turpentinegas-discharge in the sulphate process and pressure release in the sulphite process. Mainly, however, the

said discharge takes place after the digesting process has been concluded, as it is necessary to relieve the pressure somewhat before discharging (blowing) the contents of the digester.

For reasons of heat economy and efiiciency it is important that the said discharge takes place during the shortest possible time. In the known processes for digesting pulp it is, however,' very difficult to attain this object as a pressure release inside the digester has the immediate efiectof causing a more or arate vessels, the liquid being conducted from the digester through a number of liberating vessels arranged in series and with gradually decreasing pressures.

By this means the liberation may take place quickly and, at the same time thelib'erated steam and/or gas may be collected at a high temperature, so as to obtain an economic utilization of the Waste heat.

In the drawin which diagrammatically illustrate three ifierent practical embodiments of apparatus, by means of which the procem may be carried out,

Figure 1 represents one form of the invention with a smgle liberating vessel through which the digesting liquid passes;

Figure 2 represents an arrangement having serially arranged liberating tanks, and

' less violent formation of bubbles and foam,

Figure 3 represents a further modification in which the material may pass from the libcrating tank by gravityv to the digester.

In accordance with the embodiment illustrated on Fig. 1, the boiler 1 contains digested pulp at a temperature of approximately 170 (3., which has to be discharged from the digester at a temperature of approximately 140 C. corres onding to a pressure of approximately 3. atmospheres. 2 is a cylindrical vessel, which communicates with the digesteril through pipe 3, the opening of which into digester 1 is covered by a screen or sieve 4. Tube 3 opens tangentially into vessel 2.

ess is carried out as-follows:

PATENT OFFICE. I

Before discharging, the cocks or valves 11,

12 in tubes 3 and 'Z'are opened, pump 5' is started'and valve 13 connecting tube 16 with vessel 8 is opened. Digesting liquid enters vessel 2, the pressure decreases and gas and/or steam is.liberated at a temperature off. instr 150 0., which may be controlled by opening valve 13 more or less. Through this valve steam and/or gas is introduced in vessel 8, where it is accumulated or absorbed in a suitable liquid under a temperature of about 145 C. In case the heat of the steam is directly absorbed in liquid, the remaining gas is let out through tube 17 by means of valve 18. From vessel 2 the remaining liquid at a temperature of 150 C. is returned to the digester through pump 5 and tubes 6 and After this first stage of the process has been completed, the temperature of the digester instance are supposed to be s termination of stance by steam pressure introduced at the top of vessel 2 through tube 19.

and/or gas liberated has been accumulated 1n successive stages at the highest possible temperature.

It will be obvious that any number of v stages may be used in order to reduce the digester temperature and pressure successively to the values desired.

The number of stages used and the temperature interval between the several heat accumulating vessels is determined in aclustrated on Fig. 2 the digesting liquid, from which it is desired to liberate steam and/or gas with the object of reducing the pressure, is passed through a number of liberating vessels 2 2", 2, arranged in series.

Each of the liberating vessels 2, 2", 2 operates at a certain pressure and temperature, which is controlled by means of the valves 13, 14, 15, and the steam and/or gas liberated transfers its heat to a suitable medium in the vessels 8, 9, 10, which in this given the form of surface condensers. As a result of this process it will be seen that the temperature of the digesting liquid is reduced to about 140 0., whereas the heat of the steam and/or gas liberated is retained (by means of the cooling fluid passing through the surface condensers) at a temperature of 145 C.

On Fig. 3 there is diagrammatically illustrated an arrangement for turpentine-gasdischarge from the digester. In accordance with this form, the tur entine-gas and steam is let out through a tu e into liberating vessel 2 where the gas and steam is separated from the liquid carried with it, the steam and gas being let out to a suitable accumulator or the like through tube 16.

The remaining liquid is collected in vessel 2 and pressed back into the digester after the liberating process'for in- By mounting vessel 2 at a sufiiciently high level, thesteam pressure ma be dispensed with and the liquid collected in 2 may be returned to the digester by gravity.

Even with this arrangement it is obvious that a pump such as 5 may be used for returning the liquid from vessel 2.

Claims 1. In a process for digesting wood pulp in closed digesters under pressure, the method for reducing the pressure in the digester immediately before the discharging, and af-. ter the digesting process has terminated, which consists in conducting the digesting liquid from the digester to'a separate gas and steam liberating vessel under a reduced pressure liberating steam and/or gas from the digesting liquid in said vessel and returning the liquid to the digester.

2. In a process for digesting wood pulp in closed digesters under pressure, for reducing the pressure in the digester immediately before the discharging, and after the digesting process has terminated, which consists in conducting the digesting liquid from the digester to a number of separate gas and steam liberating vessels arranged in series under gradually decreased pressures, liberating steam and/or gas from the digesting liquid in said vessels and returning the liquid to the digester.

3. In a process for digest-ing wood pulp in closed digesters under pressure, the method for reducing the pressure in the digester immediately before the discharging, and after the digesting process has terminated, which consists in conducting the digesting liquid from the digester to a separate gas and steam liberating vessel under a reduced pressure liberating steam and/or gas from the digesting liquid in said vessel in a number of stages at gradually decreased pressures, accumulating the heat of the gas and/or steam liberated in suitable heat accumulating apparatus, and returning the liquid to the digester.

4. In a process "for digesting wood pulp in closed digesters under pressure, the method for reducing the pressure in the digester which consists in conducting the digesting liquid from the digester to a number of separate gas and steam liberating vessels, ar-

rangedin series under gradually decreased pressures liberating steam and/or gas from the digesting liquid in said vessel and returning the liquid to the digester, liberating steam and/or gas from the digesting liquid in said series of vessels, and'accumulating the heat of said liberated steam and/0r gas in the cooling liquid of a series of surface condensers.

In testimony whereof I have signed. my name to this specification.

EINAR MORTERUD.

the method a 

